1. Field of the Invention
The present invention concerns a defoaming apparatus for use in the aerobic fermentative production of useful chemical and biological substances, as well as methods of defoaming and controlling aerobic fermentation cultures in the production of useful chemical and biological substances.
2. Discussion of the Background
In the fermentation industry, aerobic microorganisms are generally used for producing useful chemical and biological substances. Such aerobic microorganisms are cultured in a culture tank or fermentation tank, containing a liquid culture medium. Where a large amount of air is blown into a culture liquid, foams are expected to be generated. If the generated foams are not reduced or eliminated during culturing, the tank is filled with the scum of the foams, causing the scum to overflow into the exhaust system of the culture tank. In particular, if the amount of the culture liquid is relatively large compared with the capacity of the culture tank (approximately 70% or more of the tank capacity), then the possibility of scum or foam overflowing into the exhaust system increases dramatically.
In general, methods of defoaming are employed for the purpose of inhibiting foaming during fermentative culturing. In one method, a foam detector is provided in the culture tank, and a defoaming agent (a surfactant or silicone) is added to the tank (for instance, see C. L. Kroll et al.; I.E.C., 48, 2190 (1956)). In accordance with this method, however, the amount of defoaming agent in the tank is difficult to control. Therefore, productivity using this method is low.
Another defoaming method employs defoaming blades disposed above the stirring blades in the fermentation tank, to act on areas or surfaces of vapor (air)-liquid contact. In general, as fermentation progresses, components such as nutrient sources and pH-altering substances are added to the culture medium. In accordance with the defoaming method employing defoaming blades, however, the scum of the foams generated is merely pushed back into the medium as the amount of the culture liquid in the culture tank increases. Using only the device of this method, complete control of the scum surface of the foams generated is impossible.
In fermentative production operations typically used at the commercial plant level, a combined method is used in which the scum surface of the generated foams is detected with an electrode sensor disposed in the upper area of the culture tank (above the surface of the culture medium), and a surfactant and a silicone chemical agent are added to the tank as defoaming agents. The scum surface of the generated foams is thus retained in the lower portion of the culture medium below the defoaming blades (for instance, see Japanese Patent Publication No. 46-30786, and FIG. 4 herein).
In accordance with this combined method, however, the amount of the culture liquid to be charged into the culture tank is limited due to the defoaming blades. Increasing the amount of culture liquid increases the amount of power necessary to turn the stirring and defoaming blades. Generally, when the amount of culture liquid increases to that amount requiring about three times the original power to turn the blades, vigorous foaming occurs. At this point, the power necessary to turn the blades exceeds the maximum capacity of conventional motors. As a result, the space in the upper area of the culture tank is not utilized efficiently. Further, the product output and/or the efficiency of production as a function of energy input is reduced, compared with methods which expend energy to rotate only the stirring blades.
Other defoaming means include a method of removing the scum of foams by providing a separate rotor in the upper area of the culture tank (above the surface of the liquid culture medium), and rotating the rotor by an electric motor at a high speed to remove the scum of the foams generated during fermentation (for instance, see I. H. Muller; Process Biochem., 37 (June, 1972)); and a defoaming tank having horizontally rotating blade propellers disposed at the exhaust outlet (see Japanese Utility Model Publication No. 39-36996).
The methods providing a separate rotor or horizontally rotating blade propellers have a high power cost for the electric motor when the culture tank is of sufficiently large scale. The amount of the culture liquid therein is large, proportionally creating even more foam, thus limiting the allowable amount of culture liquid in the fermentation tank.
Another defoaming method breaks foams generated in a fermentation or culture tank against a cyclone, a baffle, or the like, positioned in a "loop" external to the tank. The culture liquid from which the generated foams have been removed is then re-circulated into the culture tank (see Japanese Patent Publication Nos. 39-29800 and 39-26041). In accordance with this method, however, complete defoaming is impossible. Culture liquid to be re-circulated still contains significant amounts of foam. As a result, the gas hold-up in the culture liquid is not lowered, and the amount of the culture liquid charged into the culture tank cannot be increased, compared to the previously discussed methods. Thus, productivity is also low, as is its utility as a practical production process.
An improved defoamer introduces culture liquid and foams generated therein into one cyclone located externally to the fermentation tank, and the defoamed liquid is then recycled into the culture tank from the top and is brought into contact with a gas fed into the tank from the bottom by countercurrent contact (see Japanese Patent Laid-Open Application No. 51-142585). Using this device, however, sterilization of the defoamer is difficult during fermentative production processes, and the feed lines of the device are often clogged due to solids or the like in the culture liquid. Thus, this device is also difficult to use in an actual production process.
A defoaming method is also known in which a part of a culture liquid is "sprinkled" (see Japanese Patent Laid-Open Application No. 51-35470). However, the "sprinkling" method is complicated by microbial contamination of the sprinkling device, and complete defoaming is impossible. Thus, the desired effects of the method are not realized.
A defoaming device to be operated by discharging is also known (see Japanese Patent Laid-Open Application Nos. 55-15639). The device is useful on a small scale; for example, less than 1 kiloliter. However, on a large scale (e.g., that used for mass production), problems arise and the equipment for the device is complicated. Thus, the discharging device is not practical.
The above-mentioned known defoaming devices and methods of controlling the scum surface of foams are not applicable to practical production processes. For example, use of amounts of liquid culture media up to 60-70% (or more) of the total capacity of the culture tank is impossible without reducing the yield of the intended products.